JP7684586B2 - Hot stamped compact - Google Patents

Description

The present invention relates to a hot stamped product.
This application claims priority based on Japanese Patent Application No. 2021-022063, filed in Japan on February 15, 2021, the contents of which are incorporated herein by reference.

In recent years, there has been a demand for lighter automobile bodies from the standpoint of environmental protection and resource conservation, and the application of high-strength steel sheets to automobile body parts has accelerated. Meanwhile, automobile body parts are manufactured by press forming. As the strength of the steel sheets that make up automobile body parts increases, not only does the forming load during press forming increase, but formability also decreases. Therefore, when press forming high-strength steel sheets, formability into components with complex shapes becomes an issue. To solve this issue, the application of hot stamping technology, in which the steel sheet is heated to a high temperature in the austenite region where it softens, and then press forming is performed, is being promoted. Hot stamping is attracting attention as a technology that achieves both forming into automobile body parts and ensuring strength by performing quenching treatment in a die at the same time as press forming.

Among auto body parts, components used for shock absorption and frame deformation control are required to be less susceptible to fracture due to deformation during a collision. To prevent fractures due to deformation during a collision, auto body parts are required to have excellent bendability. In addition, the anisotropy of bendability is required to be small so that fractures can be prevented even when the parts are deformed in various deformation modes during a collision.

The bendability of a material is correlated with its tensile strength, and reducing the tensile strength improves the bendability. The main phase in the microstructure of hot stamped material is martensite, and it is known that the tensile strength of martensite is greatly affected by the C component of the steel.

For example, Patent Document 1 discloses a high-strength steel plate having an area ratio of martensite of 95% or more in the entire structure, a dissolved C content of the martensite of 0.05 mass% or less, a density of carbides having a major axis of 200 nm or more of 50 pieces/μm3 or ^less , and a tensile strength of 1270 MPa or more.

Patent Document 2 discloses a non-tempered high-tensile thick steel plate with a yield strength of 885 MPa or more, characterized in that the microstructure is a mixed structure of martensite and lower bainite, and the combined area ratio of both structures is 95% or more.

Patent Document 3 discloses a high-strength steel with excellent resistance to delayed fracture, characterized in that 70 volume % or more of the metal structure is martensite phase or tempered martensite phase, and 50 volume % or more of the martensite phase or tempered martensite phase is martensite phase or tempered martensite phase formed from unrecrystallized austenite phase.

The technologies in the above Patent Documents 1 to 3 improve various properties by specifying the martensite fraction and controlling the microstructure. However, there is no mention of martensite with controlled autotempering behavior, nor is there any mention of improving bendability or reducing the anisotropy of bendability.

Japanese Patent Application Publication No. 2018-109222 Japanese Patent Application Publication No. 2011-12315 Japanese Patent Application Publication No. 11-229075

In view of the above problems, the present invention aims to provide a hot stamped body having high strength, excellent bendability, and small anisotropy in bendability.

As a result of investigations into methods for obtaining the above-mentioned hot stamped steel, the present inventors have obtained the following findings.
In order to obtain excellent bendability in a high-strength hot stamped steel and further reduce the anisotropy of bendability, it is important to have a desired amount of soft martensite in the metal structure. Soft martensite has a low dislocation density and is produced by auto-tempering martensite.

In order to sufficiently auto-temper martensite, it is important to use steel sheet with a high martensite transformation start temperature (Ms point). This is because auto-tempering begins in the high temperature range when steel sheet with a high Ms point is hot stamped.

Furthermore, controlling the hot stamping conditions is also effective in sufficiently auto-tempering the martensite. The slower the cooling rate after hot stamping, the more likely auto-tempering occurs. In order to control the cooling rate after hot stamping, it is effective to control the surface pressure of the die during hot stamping and to control the holding time at bottom dead center.

The gist of the present invention, which has been made based on the above findings, is as follows.
[1] A hot stamped steel according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.050-0.150%,
Si: 0.010 to 1.000%,
Mn: 1.00-2.00%,
Al: 0.001-0.500%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0100% or less,
B: 0.0005 to 0.0050%,
Cr: 0 to 0.50%,
Mo: 0-0.500%,
Ni: 0-3.00%,
Cu: 0-3.00%,
Co: 0 to 0.50%,
Sn: 0-0.500%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
REM: 0-0.0050%, Sb: 0-0.0200%, and one or two selected from the group consisting of Ti: 0.005-0.100%, and Zr: 0.005-0.100%,
Contains one or two of the group consisting of Nb: 0.015 to 0.100% and V: 0.005 to 0.100%,
The balance is Fe and impurities,
The following formulas (1) and (2) are satisfied:
The metal structure contains martensite at 90% by area or more, and the martensite contains soft regions having an average grain average image quality value of 123,000 to 200,000 at 5 to 25% by area.
561-474 x C-33 x Mn-17 x Cr-17 x Ni-7.5 x Si-21 x Mo + 10 x Co>440 ... (1)
0.265×Ti+0.140×Zr>N…(2)
In the above formulas (1) and (2), the element symbols indicate the content of each element in mass %, and 0 is substituted when the element is not contained.
[2] The hot stamped steel according to the above [1], wherein the chemical composition is, in mass%,
Cr: 0.005-0.50%,
Mo: 0.005-0.500%,
Ni: 0.005-3.00%,
Cu: 0.005-3.00%,
Co: 0.005-0.50%,
Sn: 0.005-0.500%,
Ca: 0.0005-0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.0005 to 0.0050%, and Sb: 0.0005 to 0.0200%
The compound may contain one or more of the following compounds:
[3] The hot stamped steel according to the above [1], wherein the chemical composition is, in mass%,
Co: 0.005 to 0.50%,
Sn: 0.005 to 0.500%, and Sb: 0.0005 to 0.0200%
The compound may contain one or more of the following compounds:
[4] In the hot stamped steel according to any one of the above [1] to [3], in the metal structure, carbides having an average grain size of 20 to 500 nm and containing one or more elements selected from the group consisting of Nb, Ti, Zr, and V may have a number density of 0.3 to 10.0 pcs/ ^μm2 .
[5] The hot stamped steel according to any one of the above items [1] to [4] may have a plating layer on a surface thereof.

According to the above aspect of the present invention, it is possible to provide a hot stamped body having high strength, excellent bendability, and small anisotropy in bendability.

The hot stamped body according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the hot stamped body according to this embodiment will be explained.

Note that the numerical ranges described below with "~" include the lower and upper limits. Numerical values indicated as "less than" and "more than" are not included in the numerical range. All percentages for chemical composition are by mass.

The hot stamped steel according to this embodiment has a chemical composition, in mass%, of C: 0.050 to 0.150%, Si: 0.010 to 1.000%, Mn: 1.00 to 2.00%, Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, B: 0.0005 to 0.0050%, Cr: 0 to 0.50%, Mo: 0 to 0.500%, Ni: 0 to 3.00%, Cu: 0 to 3.00%, Co: 0 to 0.50%. %, Sn: 0-0.500%, Ca: 0-0.0050%, Mg: 0-0.0050%, REM: 0-0.0050%, and Sb: 0-0.0200%, and also contains one or two types selected from the group consisting of Ti: 0.005-0.100% and Zr: 0.005-0.100%, and one or two types selected from the group consisting of Nb: 0.015-0.100% and V: 0.005-0.100%, with the balance consisting of Fe and impurities.
Each element will be described in detail below.

C: 0.050-0.150%
C is an element that greatly affects the strength of a hot stamped steel. If the C content is less than 0.050%, the strength of the hot stamped steel decreases. Therefore, the C content is set to 0.050% or more. Preferably, the C content is 0.070% or more, 0.080% or more, or 0.090% or more.
On the other hand, if the C content exceeds 0.150%, the strength of the hot stamped steel becomes too high, and the bendability deteriorates. Therefore, the C content is set to 0.150% or less. Preferably, the C content is 0.140% or less, 0.130% or less, 0.120% or less, 0.110% or less, or 0.100% or less.

Si: 0.010-1.000%
Si has temper softening resistance and has the effect of suppressing the decrease in strength due to auto-tempering during hot stamp quenching. If the Si content is less than 0.010%, the above effect cannot be obtained, and the desired strength may not be obtained, and bendability may deteriorate. Therefore, the Si content is set to 0.010% or more. Preferably, it is 0.020% or more, 0.030% or more.
On the other hand, if the Si content exceeds 1.000%, a problem of surface scale occurs. That is, after the scale generated during hot rolling is pickled, a pattern due to surface unevenness occurs, and the surface appearance becomes inferior. In addition, when plating is performed on the steel sheet surface, plating property is deteriorated. Therefore, the Si content is set to 1.000% or less. Preferably, it is 0.700% or less, 0.500% or less, or 0.300% or less.

Mn: 1.00-2.00%
Mn is an element that improves the strength of a hot stamped body and the hardenability of steel. If the Mn content is less than 1.00%, sufficient strength cannot be obtained in the hot stamped body. Therefore, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.20% or more, and more preferably, 1.40% or more.
On the other hand, if the Mn content exceeds 2.00%, the above effects are saturated and the bendability is reduced. Therefore, the Mn content is set to 2.00% or less, preferably 1.80% or less, and more preferably 1.60% or less.

Al: 0.001-0.500%
Al is an element used as a deoxidizer for molten steel. If deoxidization is insufficient, the bendability of the hot stamped steel decreases due to the excessive oxides generated. In order to sufficiently deoxidize the molten steel, the Al content is set to 0.001% or more. Preferably, the Al content is 0.010% or more, and 0.030% or more.
On the other hand, if the Al content exceeds 0.500%, a large amount of nonmetallic inclusions are formed, and surface defects tend to occur in the hot stamped steel. Therefore, the Al content is set to 0.500% or less, preferably 0.300% or less, 0.200% or less, or 0.100% or less.

P: 0.100% or less P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, and the toughness and bendability of the hot stamped body are reduced. Therefore, the P content is set to 0.100% or less. Preferably, it is 0.080% or less, 0.050% or less.
Although the lower limit of the P content is not particularly specified, an excessive reduction in the P content increases the refining cost, so the P content may be set to 0.001% or more.

S: 0.0100% or less S affects nonmetallic inclusions in steel and deteriorates the bendability of hot stamped steel. Therefore, the S content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.
Although there is no particular restriction on the lower limit of the S content, an excessive reduction in the S content increases the manufacturing cost in the desulfurization step, and therefore the S content may be set to 0.0001% or more.

N: 0.0100% or less N is an impurity element that forms nitrides in steel that become bending crack initiation points, thereby deteriorating the bendability of the hot stamped body. If the N content exceeds 0.0100%, coarse nitrides are formed in the steel, and the bendability of the hot stamped body is significantly reduced. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less.
Although there is no particular lower limit for the N content, reducing it to less than 0.0001% is economically undesirable because the denitrification cost increases significantly. In practical operation, the N content may be 0.0001% or more, or 0.0005% or more.

B: 0.0005-0.0050%
B has the effect of improving the hardenability during hot stamping or during cooling after hot stamping, thereby improving the strength of the hot stamped body. If the B content is less than 0.0005%, the above effect cannot be obtained. Therefore, the B content is set to 0.0005% or more. Preferably, the B content is 0.0007% or more, and 0.0010% or more.
On the other hand, if the B content exceeds 0.0050%, cracks may occur during hot rolling, the above effect may saturate, and borides may cause a decrease in bendability. Therefore, the B content is set to 0.0050% or less, and preferably 0.0030% or less.

One or two of the group consisting of Ti: 0.005-0.100% and Zr: 0.005-0.100% Ti and Zr have the effect of forming carbonitrides in steel and improving the strength of the hot stamped body by precipitation strengthening. Furthermore, they have the effect of fixing N as nitrides, suppressing BN generation, and expressing the hardenability improving effect of B. In order to obtain these effects, one or more of Ti: 0.005% or more and Zr: 0.005% or more are contained. It is not necessary to contain both Ti and Zr, and any one of them may be contained in the above content. As long as any one of Ti and Zr is contained in the above content, the other elements may be contained in less than the above content or may not be contained. The content of the above elements is preferably one or more of Ti: 0.010% or more and Zr: 0.010% or more.
On the other hand, if the content of any one of these elements exceeds 0.100%, a large amount of carbonitrides is generated, which deteriorates the bendability of the hot stamped steel. Therefore, the contents of Ti and Zr are each set to 0.100% or less, and preferably 0.080% or less.

One or two of the group consisting of Nb: 0.015-0.100% and V: 0.005-0.100% Nb and V have the effect of forming carbonitrides in steel and improving the strength of the hot stamped body by precipitation strengthening. Furthermore, as a solid solution element, they have the effect of refining the structure and improving the strength and bendability of the hot stamped body. In order to obtain these effects, one or more of Nb: 0.015% or more and V: 0.005% or more are contained. It is not necessary to contain both Nb and V, and any one of them may be contained in the above content. As long as any one of Nb and V is contained in the above content, the other elements may be contained in less than the above content or may not be contained. The content of the above elements is preferably one or more of Nb: 0.020% or more and V: 0.010% or more.
On the other hand, if the content of any one of these elements exceeds 0.100%, a large amount of carbonitrides is generated, which deteriorates the bendability of the hot stamped steel. Therefore, the contents of Nb and V are each set to 0.100% or less, and preferably 0.080% or less.

561-474 x C-33 x Mn-17 x Cr-17 x Ni-7.5 x Si-21 x Mo + 10 x Co>440 ... (1)
0.265×Ti+0.140×Zr>N…(2)
The chemical composition of the hot stamped steel according to this embodiment satisfies formula (1) and formula (2). Note that the element symbols in formula (1) and formula (2) indicate the content of each element in mass%, and 0 is substituted when the element is not contained.

The left side of formula (1) is a formula for calculating the Ms point (°C). If the left side of formula (1) is 440°C or less, the Ms point will be low, and even if hot stamping is performed under favorable conditions, the desired amount of soft region cannot be obtained. Therefore, the left side of formula (1) (Ms point) is set to be greater than 440°C. The left side of formula (1) is preferably 450°C or more, more preferably 460°C or more. The upper limit of the left side of formula (1) is not particularly specified, but may be 600°C or less, 550°C or less, or 500°C or less.

The left side of formula (2) is a formula for calculating the amount of nitrogen (mass%) fixed to the nitride containing Ti and Zr. If the left side of formula (2) is equal to or less than the N content, BN is generated and the hardenability improving effect of B cannot be sufficiently obtained. Therefore, the left side of formula (2) is made to exceed the N content.
The upper limit of the left side of the formula (2) is not particularly specified, but may be set to 0.150% or less.

The balance of the chemical composition of the hot stamped body according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and/or during the steelmaking process and are permissible to the extent that they do not impair the properties of the hot stamped body according to this embodiment.

The hot stamped steel according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. If the following optional elements are not contained, the content is 0%.

Cr: 0.005 to 0.50%, Mo: 0.005 to 0.500%, Ni: 0.005 to 3.00%, and Cu: 0.005 to 3.00%
Cr, Mo, Ni, and Cu are elements that improve the hardenability of steel and have the effect of improving the strength of hot stamped bodies. Therefore, one or more of these elements may be contained as necessary. In order to ensure that this effect is exerted, it is preferable that the content of at least one of Cr, Mo, Ni, and Cu is 0.005% or more.
On the other hand, if the Cr content exceeds 0.50%, the Mo content exceeds 0.500%, or the Ni or Cu content exceeds 3.00%, the carbides present after hot rolling, cold rolling, or annealing (including after plating) are stabilized, which may delay the dissolution of the carbides during heating during hot stamping and reduce hardenability. Therefore, the Cr content is set to 0.50% or less, the Mo content is set to 0.500% or less, and the Ni and Cu contents are each set to 3.00% or less.

Co:0.005~0.50%
Co is an element that has the effect of increasing the Ms point and improving the bendability of the hot stamped steel. Therefore, Co may be contained as necessary. In order to reliably exert the above effect, the Co content is preferably 0.005% or more.
On the other hand, if the Co content exceeds 0.50%, the hardenability of the steel decreases, so the Co content is set to 0.50% or less.

Sn: 0.005-0.500%
Sn has the effect of improving the corrosion resistance of the hot stamped steel, and may be contained as necessary. In order to ensure that this effect is exhibited, the Sn content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.
On the other hand, even if the Sn content exceeds 0.500%, the above effect is saturated, so the Sn content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.150% or less.

Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.0005-0.0050%
Ca, Mg and REM have the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping. Therefore, one or more of these elements may be contained as necessary. In order to ensure the above effect, it is preferable that the content of at least one of Ca, Mg and REM is 0.0005% or more.
On the other hand, if the content of Ca, Mg or REM exceeds 0.0050%, the effect of refining inclusions in the steel is saturated and the alloy cost increases. Therefore, the content of Ca, Mg and REM is set to 0.0050% or less.
In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM refers to the total content of these elements.

Sb: 0.0005-0.0200%
Sb may be contained as necessary to suppress decarburization during hot rolling. By containing Sb, decarburization can be suppressed during hot rolling and hot stamping when a non-plated cold rolled steel sheet is used. In order to ensure this effect, the Sb content is preferably 0.0005% or more.
On the other hand, if the Sb content exceeds 0.0200%, the above effects are saturated, so the Sb content is set to 0.0200% or less.

In the chemical composition of the hot stamped body according to this embodiment, in order to improve the characteristics of the hot stamped body, it is preferable to contain one or more of the group consisting of Co, Sn and Sb among the optional elements mentioned above in the above-mentioned amounts.

The chemical composition of the above-mentioned hot stamped body may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. If the hot stamped body has a plating layer on its surface, the plating layer on the surface may be removed by mechanical grinding before analyzing the chemical composition.

Next, the metal structure of the hot stamped steel according to this embodiment will be described.
The metal structure of the hot stamped steel according to this embodiment contains martensite at 90% by area or more, and the martensite contains soft regions at 5 to 25% by area.

Martensite: 90% by area or more Martensite is a structure necessary for obtaining a desired strength. If the martensite is less than 90% by area, the desired strength cannot be obtained. Therefore, the martensite is set to 90% by area or more. Preferably, the martensite is 93% by area or more, 95% by area or more, or 97% by area or more. The martensite may be 100% by area.

In this embodiment, the remaining structure other than martensite may include ferrite, pearlite, upper bainite, lower bainite, and retained austenite. The total of these remaining structures is preferably 10 area% or less in relation to the area ratio of martensite. The total of the remaining structures is preferably 7 area% or less, 5 area% or less, 3 area% or less, or may be 0 area%.

Soft region: 5 to 25% by area of martensite
The soft region is a region of martensite that has a low dislocation density and relatively low strength. In this embodiment, the soft region refers to a region having an average grain average image quality value (average GAIQ value) of 123,000 to 200,000.

If the soft region of the martensite is less than 5% by area, the desired bendability cannot be obtained, and the anisotropy of the bendability cannot be reduced. Therefore, the soft region of the martensite is set to 5% by area or more, preferably 10% by area or more, and more preferably 15% by area or more.
On the other hand, if the soft region of the martensite exceeds 25% by area, the desired strength cannot be obtained and the anisotropy of bending property cannot be reduced. Therefore, the soft region of the martensite is set to 25% by area or less, preferably 23% by area or less, more preferably 20% by area or less.

The mechanism by which the soft martensite phase improves bendability and reduces the anisotropy of bendability is still unclear, but the present inventors speculate as follows.
Since the soft regions in martensite are softer than the surroundings, plastic deformation is likely to occur during bending deformation, but it is believed that the high fracture limit suppresses crack generation. Furthermore, it is believed that there is an effect of suppressing crack growth in crack propagation. Therefore, it is presumed that the bendability is improved. In addition, the anisotropy of bendability is usually affected by inclusions stretched in the rolling direction. In the hot stamped steel according to this embodiment, the soft regions are uniformly dispersed, which is believed to alleviate the effect of the inclusions. As a result, it is believed that the anisotropy of bendability can be reduced.

The area ratio of martensite is measured by the following method.
A sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot stamped product (if sampling from this position is not possible, a position avoiding the end) so that the cross section of the sheet thickness can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

The cross section of the sample is etched with LePeller's reagent. The cross section etched with LePeller's reagent is observed at 1/4 of the plate thickness (the area from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) at 500x magnification in 10 fields of view to obtain optical microscope photographs. The obtained optical microscope photographs are analyzed using image analysis software "Photoshop CS5" manufactured by Adobe, and the area ratio of martensite is determined.

As an image analysis method, the maximum brightness value _Lmax and the minimum brightness value _Lmin of the image are obtained from the image, and the area ratio of martensite, which is the white area, is defined as a portion having pixels with brightness from _Lmax -0.3 ( _{Lmax -Lmin} ) to _Lmax , the area ratio of martensite, which is the white area, is calculated by defining the area ratio of martensite from _Lmin to _Lmin +0.3 ( _{Lmax -Lmin} ), and the other areas as gray areas. Image analysis is performed in the same manner as above for a total of 10 observation fields to measure the area ratio of martensite. The average value of the obtained area ratios is calculated, and this average value is regarded as the area ratio of martensite. This gives the area ratio of martensite.
The area ratio of the remaining structure is obtained by subtracting the area ratio of martensite from 100%.

The area ratio of the soft region is measured by the following method.
A sample is cut out from a position 50 mm or more away from the end face of the hot stamped body (a position that avoids the end if it is not possible to sample from this position) so that the plate thickness cross section can be observed. The plate thickness cross section of this sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the sample is polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the strain introduced into the surface layer of the sample.

At any position in the longitudinal direction of the thickness cross section of the sample, a length of 50 μm and a position at 1/4 of the thickness (a region from 1/8 of the thickness from the surface to 3/8 of the thickness from the surface) are measured at measurement intervals of 0.1 μm by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62.

The obtained crystal orientation information is used to obtain a Grain Average Image Quality map (GAIQ map) using the "OIM Data Collection" function of the software attached to the EBSD analyzer and the "Grain Average Misorientation" function installed in "OIM Analysis (registered trademark)". In the obtained GAIQ map, the area surrounded by the interface with a crystal orientation difference of 5° or more is defined as a crystal grain, and the area ratio of the area in the unit crystal grain where the average GAIQ value is 123,000 to 200,000 is calculated. For a total of 10 observation fields, the area ratio of the area in the unit crystal grain where the average GAIQ value is 123,000 to 200,000 is calculated. The average value of the obtained area ratios is calculated, and this average value is regarded as the area ratio of the soft region. The area ratio of the soft region thus obtained is divided by the area ratio of martensite obtained by the above-mentioned method (area ratio of soft region/area ratio of martensite×100) to obtain the area ratio of the soft region in the martensite.

In the metal structure according to the present embodiment, the number density of the carbides containing one or more of the group consisting of Nb, Ti, Zr and V may be 0.3 to 5.0 pieces/μm ^{2, and the average grain size is 20 to 500 nm. By setting the number density of the carbides containing one or more of the group consisting of Nb, Ti, Zr and V to 0.3 to 5.0 pieces/μm 2 ,} the bendability of the hot stamped body can be further improved. The precipitation state of the carbides contributes to crack propagation during bending deformation. It is considered that the larger the size of the carbides and the lower the number density, the lower the bendability.

The number density of the carbides is measured by the following method.
A sample is cut out from any position 50 mm or more away from the end face of the hot stamped body (if it is not possible to sample from this position, a position that avoids the end) so that the plate thickness cross section can be observed. The observation surface is mirror-polished, and then the base steel is dissolved by constant-potential electrolytic etching to reveal the precipitate. The conditions for the constant-potential electrolytic etching are a mixture of 10 volume % acetylacetone, 1 volume % tetramethylammonium chloride, and the remainder methyl alcohol as the electrolyte, a set potential of -200 mV, and a coulomb amount of 10 c/ ^cm2 .

Then, at the 1/4 position of the plate thickness (the region from the surface to the depth of 1/8 of the plate thickness to the surface to the depth of 3/8 of the plate thickness), 10 or more fields of view are observed at a magnification of 20,000 times. The composition of the precipitates can be measured by EDS (energy dispersive X-ray spectroscope). Among the precipitates, carbides containing one or more of the group consisting of Nb, Ti, Zr and V are selected, and the major axis and minor axis of each carbide are measured from the observation photograph. The average value of the major axis and minor axis is calculated, and the average value is regarded as the average grain size of the carbides. The number of carbides having an average grain size of 20 to 500 nm is counted and divided by the measured area to obtain the number density of carbides having an average grain size of 20 to 500 nm and containing one or more of the group consisting of Nb, Ti, Zr and V.
When EDS analysis of a precipitate detects one or more elements selected from the group consisting of Nb, Ti, Zr, and V, and C, the precipitate is regarded as a carbide containing one or more elements selected from the group consisting of Nb, Ti, Zr, and V.

The hot stamped product according to the present embodiment may have a plating layer on its surface in order to further improve corrosion resistance. Examples of the plating layer include an Al-based plating layer such as a hot-dip aluminum plating layer or an aluminum-zinc plating layer, a hot-dip galvanized layer, a galvannealed layer, an electrolytic galvanized layer, and a zinc-nickel plating layer.

The plating layer may be disposed on either one surface or both surfaces of the hot stamped body. Although there are no particular limitations on the coating weight, it is preferable that the Al-based plating layer is 15 to 120 g/m ² per side, the hot-dip galvanized layer is 30 to 120 g/m ² per side, the alloyed hot-dip galvanized layer is 30 to 120 g/m ² per side, and the electrogalvanized layer and zinc-nickel plating layer are 5 to 100 g/m ² per side.

In this embodiment, the Al-based plating layer refers to a plating layer containing 50% or more by mass of Al. Elements other than Al may include Si: 0.1 to 20% by mass, Fe: 0.1 to 10% by mass, Zn: 0.1 to 45% by mass, and the balance (Cu, Na, K, Co, Ni, Mg, etc.): less than 0.5% by mass.

In this embodiment, the Zn-based plating layer refers to a plating layer containing 50% or more by mass of Zn. Elements other than Zn may include Si: 0.01 to 20% by mass, Fe: 0.1 to 10% by mass, Al: 0.01 to 45% by mass, and the balance (Cu, Na, K, Co, Ni, Mg, etc.): less than 0.5% by mass.

The composition of the plating layer is analyzed by the following method.
A sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot stamped product (if sampling from this position is not possible, a position avoiding the end) so that the cross section of the sheet thickness can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

The above sample is embedded in resin and polished, and then the layer structure of the plate thickness cross section is observed with a scanning electron microscope (SEM). Specifically, the SEM is used at a magnification that allows the steel plate and plating layer to fit within the observation field. For example, by observing a backscattered electron composition image (COMPO image), it is possible to infer how many layers the cross-sectional structure is made up of.

Next, an electron probe microanalyzer (EPMA) is used to map and analyze an area of 50 μm in the sheet surface direction and plating layer thickness + 30 μm in the sheet thickness direction. If the plating layer is an Al-based plating layer, the average values of the Fe concentration and Al concentration in the sheet surface direction are determined. Next, the relationship between the sheet thickness position and the Al concentration, and the relationship between the sheet thickness position and the Fe concentration are determined. The sheet thickness position where the Al concentration and Fe concentration are the same as the Al concentration and Fe concentration of the steel sheet can be determined to be the interface between the steel sheet and the Al-based plating layer. The Al concentration and Fe concentration of the steel sheet referred to here are obtained by measurement using EPMA.

In addition, if the plating layer is a Zn-based plating layer, the average values of the Fe concentration and Zn concentration in the sheet surface direction are determined. Next, the relationship between the sheet thickness position and the Zn concentration, and the relationship between the sheet thickness position and the Fe concentration are determined. The sheet thickness position where the Zn concentration and Fe concentration are the same as the Zn concentration and Fe concentration of the steel sheet can be determined to be the interface between the steel sheet and the Zn-based plating layer. The Zn concentration and Fe concentration of the steel sheet referred to here are obtained by measurement using an EPMA.

Plate Thickness The plate thickness of the hot stamped steel according to this embodiment is not particularly specified, but may be 0.5 to 3.5 mm from the viewpoint of reducing the vehicle body weight.

Tensile Strength The hot stamped steel according to the present embodiment preferably has a tensile strength of 980 MPa or more in order to enhance the effect of reducing the weight of the vehicle body. On the other hand, if the tensile strength is too high, the bendability decreases, so the tensile strength is preferably 1380 MPa or less.
The tensile strength is determined by preparing a No. 5 test piece described in JIS Z 2241:2011 and following the test method described in JIS Z 2241:2011.

Next, we will describe the manufacturing method of the hot stamped steel according to this embodiment. First, we will explain the manufacturing method of the hot stamping steel sheet to be subjected to hot stamping.

First, a hot-rolled steel sheet is obtained by hot rolling. The steel piece (steel material) to be subjected to hot rolling may be a steel piece produced by a conventional method, for example, a steel piece produced by a general method such as a continuous casting slab or a thin slab caster. The steel piece having the above-mentioned chemical composition is subjected to hot rolling. In order to preferably control the number density of carbides containing one or more of the group consisting of Nb, Ti, V and Zr, it is preferable that the heating temperature before hot rolling is 1200°C or higher, the coiling temperature is 600°C or lower, and the time from the completion of finish rolling to the start of coiling is 5 seconds or more.

By setting the heating temperature at 1200°C or higher, carbides containing one or more of the group consisting of Nb, Ti, V and Zr can be dissolved, and the carbides can be finely precipitated during rolling. There is no particular upper limit to the heating temperature, but it may be set to 1400°C or lower from the viewpoint of productivity.

By setting the winding temperature at 600°C or less, the number density and average particle size of carbides containing one or more of the group consisting of Nb, Ti, V and Zr can be preferably controlled. The lower limit of the winding temperature is not particularly specified, but it may be set to 400°C or more from the viewpoint of productivity.

If the time from the completion of finish rolling to the start of winding is short, the temperature will rise due to transformation heat when the coil is wound, and it may not be possible to preferably control the number density and average particle size of carbides containing one or more of the group consisting of Nb, Ti, V, and Zr. Therefore, it is preferable to set the time from the completion of finish rolling to the start of winding to 5 seconds or more. There is no particular upper limit, and it is sufficient to set it so that winding can be performed at the above-mentioned winding temperature, taking into account the sheet passing speed and cooling speed.

Next, the resulting hot-rolled steel sheet is uncoiled, pickled, and then cold-rolled. The cumulative reduction during cold rolling may be within a range that does not impede productivity, for example, 30 to 80%. This results in a cold-rolled steel sheet.

The obtained cold-rolled steel sheet may be annealed to soften it. After annealing, it is preferable to perform temper rolling. The reduction ratio in temper rolling of the steel sheet may be within a range that does not impede productivity, and may be 2% or less. A tension leveler may be used to correct the shape.

Cold-rolled steel sheets may be coated with Al-based plating, such as aluminum plating and aluminum-zinc plating, or Zn-based plating, as necessary. Although the main components of the plating are aluminum and zinc, elements such as Ni may be added to improve corrosion resistance. The plating may also contain elements such as iron as impurities.

Plating may be applied under normal plating conditions. For aluminum plating, a suitable bath concentration of Si is 5-12 mass%, with the remainder being aluminum and less than 0.5% impurities. For aluminum-zinc plating, a suitable bath concentration of Zn is 40-50 mass%, with the remainder being aluminum and less than 0.5% impurities. There is no particular problem with Mg and Zn being mixed in the aluminum plating, or with Mg being mixed in the aluminum-zinc plating. The atmosphere during plating may be normal plating conditions, whether the continuous plating equipment has a non-oxidizing furnace or does not have a non-oxidizing furnace. For zinc plating, methods such as hot-dip galvanizing, electrolytic galvanizing, and alloyed hot-dip galvanizing may be used.

A metal pre-plating may be applied to the surface of the steel sheet before plating. Examples of the metal pre-plating include Ni pre-plating, Fe pre-plating, and other metal pre-plating that improves plating properties. There is no particular problem even if a different type of metal plating or a coating of an inorganic or organic compound is applied to the surface of the plating layer.
By the above method, a steel sheet for hot stamping is obtained.

Next, the hot stamped steel sheet obtained by the above-mentioned method is subjected to, for example, the following hot stamping conditions to produce the hot stamped body according to this embodiment.

The steel sheet for hot stamping is heated to a temperature range of _Ac3 transformation point to 1000°C, and is held at that temperature range for 0.1 to 30.0 minutes, and then is quickly transported onto a die for hot stamping. Thereafter, the steel sheet is pressed and cooled in the die by heat transfer between the steel sheet and the die.

The steel sheet temperature may be varied or may be constant in the temperature range from the _Ac3 transformation point to 1000° C. _{The Ac3} transformation point can be calculated by the following formula.

_Ac3 transformation point (°C) = exp(X) + 31.5 x Mo-28
X=6.8165-0.47132×C-0.057321×Mn+0.0660261×Si-0.050211×Cr+0.10 593×Ti+2.0272×N+1.0536×S-0.12024×Si×C+0.11629×Cr×C+0.29225×C ² +0.01566×Mn ² +0.017315×Cr ²
In the above formula, the symbol for an element indicates the content of the element in mass %, and 0 is substituted when the element is not contained.

The cooling rate after hot stamping can be controlled favorably by setting the contact pressure of the die during hot stamping to the smaller value ( _Pmax (MPa)) of P _a (MPa) represented by the following formula (3) and 200 MPa, thereby obtaining a desired amount of soft region.
The lower limit of the contact pressure of the die during hot stamping is not particularly limited, but may be 0.1 MPa or more. If the contact pressure is small, the heat transfer between the die and the steel sheet becomes insufficient, and the cooling rate required for martensitic transformation cannot be obtained.
In addition, the surface pressure of the die during hot stamping can be controlled by changing the press load when the die is held at the bottom dead point. Since it is difficult to calculate an accurate surface pressure for an actual part shape, a numerical simulation of press forming may be used to determine the press load when the die is held at the bottom dead point. If the part shape is simple and the load in the normal direction to the part surface can be calculated from the press load, the surface pressure may be calculated by dividing the load in the normal direction by the surface area of the part.

P _a =8×10 ⁻¹⁷⁷ ×Ms ^67.08 …(3)
Ms in the above formula (3) is the Ms point (martensitic transformation start temperature) represented by the left side of the above formula (1).

In addition, by controlling the holding time at the bottom dead center during hot stamping, the cooling rate after hot stamping can be preferably controlled. As a result, the desired amount of soft region can be obtained. When holding at the bottom dead center during hot stamping, the steel sheet is quenched by heat transfer between the steel sheet and the die, causing martensitic transformation. The temperature (removal temperature) when the hot stamped body is removed from the die is preferably equal to or lower than the temperature at which martensitic transformation is completed. Therefore, the temperature when the hot stamped body is removed from the die is preferably 250°C or lower. The removal temperature here refers to the surface temperature of the hot stamped body when it is removed from the die.

Even if the holding time at the bottom dead center is short, if the removal temperature is low, the cooling rate of the steel sheet held in the mold will be high, and it may be difficult to obtain a soft region. Therefore, it is desirable to set the holding time at the bottom dead center relative to the removal temperature to be equal to or longer than t (seconds) obtained by the following formula (5) or (6).

When the removal temperature is less than 150° C., t=3.15×10 ⁸⁷ ×M _s ^−32.7 …(5)
When the removal temperature is 150 to 250°C, t = 2.20 × 10 ⁸⁷ × M _s ^-32.7 ... (6)
Ms in the above formulas (5) and (6) is the Ms point (martensitic transformation start temperature) represented by the left side of the above formula (1). The higher the Ms point, the easier it is to obtain a soft region, so it is considered that the holding time at the bottom dead center may be short.

By the above-described method, the hot stamped steel according to this embodiment can be manufactured.
It is preferable not to perform tempering after hot stamping. Specifically, it is preferable not to heat the material to a temperature range of 250° C. or higher for 15 minutes or longer. If tempering is performed in this manner, the soft region in the martensite becomes excessive, making it impossible to obtain the desired strength.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Cold-rolled steel sheets and plated steel sheets with thicknesses of 0.5 to 3.5 mm were manufactured by hot rolling, pickling, and cold rolling using steel pieces having the chemical compositions shown in Tables 1A to 1C, and then continuous annealing or continuous hot-dip plating was performed as necessary. The heating temperatures before hot rolling were as shown in Tables 2A to 2C, and the cumulative reduction in cold rolling was 30 to 80%.

The "types of plating layers" listed in Tables 3A to 3C are as follows:
CR: No plating layer GI: Hot-dip galvanized layer (target coating weight 60 g/ ^m2 on one side, plating on both sides)
GA: galvannealed layer (target coating weight 45 g/m ² on one side, double-sided plating)
AL: Al-based plating layer (target coating weight 80 g/m2 on ^one side, double-sided plating)
EG: electrolytic zinc plating layer (target coating amount 40 g/m ² on one side, double-sided plating)

The produced cold-rolled steel sheets and plated steel sheets were subjected to hot stamping under the conditions shown in Tables 2A to 2C. In hot stamping, flat steel sheets were sandwiched between water-cooled dies and pressed to easily prepare test pieces for tensile tests and metallographic observations. As a result, hot stamped bodies shown in Tables 3A to 3C were obtained. Production No. 77 was tempered by heating at 300°C for 20 minutes after hot stamping.
Next, the metal structure was observed and the tensile strength was measured by the above-mentioned methods.

If the obtained tensile strength was 980 MPa or more, it was judged to have high strength and to have passed the test. On the other hand, if the obtained tensile strength was less than 980 MPa, it was judged to have not high strength and to have failed the test. Also, if the obtained tensile strength was more than 1380 MPa, it was judged to have too high strength and to have failed the test.

Based on the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry, the bendability of the hot stamped body was evaluated by the following method. In this example, the displacement at the maximum load obtained in the bending test was converted to an angle according to the VDA standard to determine the bend angle α (°).

The bending test conditions were as follows:
Test piece dimensions: 60 mm (rolling direction) x 30 mm (direction parallel to the plate width direction)
Test piece thickness: 1.6 mm
Bending ridge: Parallel to rolling direction, 45° direction, perpendicular direction Test method: Roll support, punch pressing Roll diameter: φ30 mm
Punch shape: tip R = 0.4 mm
Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm
Push-in speed: 20 mm/min
Testing machine: SHIMADZU AUTOGRAPH 20kN

From the bending test in three directions, the average bending angle α _m (°) and bending anisotropy Δα (%) were determined as follows.
α _m = (α _L +2×α _D + α _C )/4
Δα=(α _m - α _min )/α _m ×100
α _L : α when bent along an axis parallel to the rolling direction
α _D : α when bent at an axis of 45° to the rolling direction
α _C : α when bent around an axis perpendicular to the rolling direction
α _min : The minimum value of α _L, α _{D, and} α _C

If the rolling direction is unknown, tests may be performed in five directions at intervals of 22.5°, and the direction in which the minimum value of the bending angle α is obtained may be regarded as the rolling direction to determine α _L , α _D , and α _C. If the rolling direction is still unknown by this method, tests may be performed in ten directions at intervals of 11.25°, and the direction in which the minimum value of the bending angle α is obtained may be regarded as the rolling direction to determine α _L , α _D , and α _C.

_αm is affected by the plate thickness and tensile strength. _It is also affected by the plating layer of the hot stamped body. When aluminum plating is used, a hard Fe-Al alloy layer is formed on the surface by the heating of hot stamping, so _αm is lower than that of GA, GI, CR (which is shot blasted after hot stamping) or EG.

In this example, when _αm was equal to or greater than the lower limit shown in Table 4, the bendability was judged to be excellent and passed. On the other hand, when _αm was less than the lower limit shown in Table 4, the bendability was judged to be poor and passed.
In addition, when Δα was 15% or less, the anisotropy of bending property was deemed small and the sample was judged as pass, whereas when Δα was more than 15%, the anisotropy of bending property was deemed large and the sample was judged as fail.

In addition, the corrosion resistance was evaluated according to the method specified in JASO M609-91 established by the Society of Automotive Engineers of Japan. Specifically, the evaluation was performed according to the following method.

A sample was taken from the hot stamped compact, and a linear scratch 70 mm long was made with a cutter on the flat surface of the sample, which had a 15 μm thick electrocoated coating, and then subjected to a cyclic corrosion test. After 120 cycles, the sample was taken out and immersed in a commercially available coating remover for 30 minutes, and the coating was then removed with a brush. The sample was then immersed in a 5% by volume aqueous solution of ammonium citrate containing an inhibitor for steel sheets, and the rust formed on the corroded area was removed with a brush. Using a Keyence VHX-7000 digital microscope, the center of the 70 mm long scratch was used as the boundary, and the thickness reduction from the reference plane was measured every 35 mm from both ends of the scratch, and the maximum value for each was obtained. The reference plane was the surface of the uncorroded area after the coating was removed, regardless of whether it was plated or not. The average value of the two maximum thickness reductions obtained was calculated.

The average value of the maximum thickness reduction obtained was evaluated according to the following criteria. In the case of AL-plated steel sheets, if the evaluation was E1, E2, or V1, it was judged that the hot stamped steel had particularly excellent corrosion resistance. In the case of GA, GI, or EG-plated steel sheets, if the evaluation was V2 or higher, it was judged that the hot stamped steel had particularly excellent corrosion resistance. In the case of CR cold-rolled steel sheets, if the evaluation was G or higher, it was judged that the hot stamped steel had particularly excellent corrosion resistance.
The following evaluations indicate that the corrosion resistance is more excellent in the order of B, G, V2, V1, E2, and E1.

E1 (Excellent-1): Less than 0.03 mm E2 (Excellent-2): 0.03 mm or more, less than 0.05 mm V1 (Very Good-1): 0.05 mm or more, less than 0.07 mm V2 (Very Good-2): 0.07 mm or more, less than 0.10 mm G (Good): 0.10 mm or more, less than 0.15 mm B (Bad): 0.15 mm or more

The above results are shown in Tables 3A to 3C. It can be seen from Tables 3A to 3C that in the examples of the present invention, hot stamped bodies having high strength and excellent bendability and small anisotropy in bendability were obtained. On the other hand, it can be seen that in the comparative examples, one or more of the above properties did not satisfy the pass criteria.

According to the above aspect of the present invention, it is possible to provide a hot stamped body having high strength, excellent bendability, and small anisotropy in bendability.

Claims (5)

The chemical composition, in mass%, is
C: 0.050-0.150%,
Si: 0.010 to 1.000%,
Mn: 1.00-2.00%,
Al: 0.001-0.500%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0100% or less,
B: 0.0005 to 0.0050%,
Cr: 0 to 0.50%,
Mo: 0-0.500%,
Ni: 0-3.00%,
Cu: 0-3.00%,
Co: 0 to 0.50%,
Sn: 0-0.500%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0050%, and Sb: 0 to 0.0200%
and one or two selected from the group consisting of Ti: 0.005 to 0.100% and Zr: 0.005 to 0.100%,
Contains one or two of the group consisting of Nb: 0.015 to 0.100% and V: 0.005 to 0.100%,
The balance is Fe and impurities,
The following formulas (1) and (2) are satisfied:
A hot stamped steel, characterized in that the metal structure contains 90 area % or more of martensite, and the martensite contains 5 to 25 area % of soft regions having an average grain average image quality value of 123,000 to 200,000 .
561-474 x C-33 x Mn-17 x Cr-17 x Ni-7.5 x Si-21 x Mo + 10 x Co>440 ... (1)
0.265×Ti+0.140×Zr>N…(2)
In the above formulas (1) and (2), the element symbols indicate the content of each element in mass %, and 0 is substituted when the element is not contained. The chemical composition, in mass%,
Cr: 0.005-0.50%,
Mo: 0.005-0.500%,
Ni: 0.005-3.00%,
Cu: 0.005-3.00%,
Co: 0.005-0.50%,
Sn: 0.005-0.500%,
Ca: 0.0005-0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.0005 to 0.0050%, and Sb: 0.0005 to 0.0200%
The hot stamped product according to claim 1, characterized in that it contains one or more of the group consisting of The chemical composition, in mass%,
Co: 0.005-0.50%,
Sn: 0.005 to 0.500%, and Sb: 0.0005 to 0.0200%
The hot stamped product according to claim 1, characterized in that it contains one or more of the group consisting of The hot stamped body according to any one of claims 1 to ³ , characterized in that, in the metal structure, carbides having an average grain size of 20 to 500 nm and containing one or more elements selected from the group consisting of Nb, Ti, Zr, and V have a number density of 0.3 to 10.0 pieces/μm2. The hot stamped body according to any one of claims 1 to 4, characterized in that it has a plating layer on its surface.